The present invention relates to a method for purging a medical fluid gas administration system of undesired fluids. The invention may find use in a variety of applications such as purging a therapeutic nitric oxide (NO) gas administration system of nitrogen dioxide (NO2) or purging a diagnostic sulfur hexafluoride (SF6) administration system to ensure proper administration of such a gas to a patient.
In connection with the former application, is it now widely recognized that the administration of NO gas to a patient increases pulmonary capillary vasodilatation. The NO gas is administered by placing it in the breathing gases inhaled by the patient, typically in very small concentrations of, for example, 0.5 nanomoles to 5.0 micromoles of NO per breath. The increase in vasodilatation improves blood-gas exchange between breathing gases and the blood providing efficacious treatment for respiratory and other diseases. The NO gas is usually supplied from a high pressure cylinder through a pressure regulator to a gas administration system. The gas administration system establishes the NO dosage and supplies same for mixing with the breathing gases prior to, or during, inhalation. The NO so supplied may be mixed with the breathing gases to provide a generally constant concentration of NO in the inhaled breathing gases. Or, the NO may be A provided to the inhaled breathing gases as short pulse doses of gas, one of which is delivered in each inhalation, usually at the beginning of inhalation.
In many cases the administration of NO is carried out when the patient is on a respirator that assists or supplants the patients own breathing action.. The respirator is connected to the patient by a breathing circuit comprised of a inspiratory limb, a Y-piece connector, an expiratory limb, and a patient limb. The inspiratory limb extends from an inspiration outlet of the respirator to the Y-piece connector. The expiratory limb connects the Y-piece connector with the respirator expiration inlet. The patient limb connects the Y-piece connector to an endotracheal tube or a breathing mask forming a conduit from the Y-piece connector to the patient""s airways and lungs. When a respirator is in use, the NO gas is delivered to the patient limb or inspiratory limb of the breathing circuit to be provided to the patient during the inspiratory phase of the patient""s respiratory cycle.
In other cases, the NO gas is provided to a spontaneously breathing patient through a face mask or nasal appliance.
A problem in the administration of NO is that NO reacts with oxygen to form nitrogen dioxide (NO2). Nitrogen dioxide is toxic even at very low concentrations and human exposure, for example in an occupational setting, is usually limited to 2-3 parts per million. For patients, who typically are already ill, there should be almost no exposure to NO2. The amount of NO2 formed during the reaction depends on the concentrations of oxygen and NO that are present and the time during which the two gases are in contact with each other.
In an NO gas administration system, NO2 can form when air and NO are both present in the gas delivery conduits of the system. The presence of air can arise during the connection of the apparatus to the high pressure NO source cylinder and pressure regulator, through leakage or migration of air into the system during and between uses, due to gas diffusion through the materials of the system when in long term storage, and for other reasons. For example, when the cylinder pressure regulator is connected to the NO supply cylinder, the internal volume of the pressure regulator contains air at ambient pressure. On connection to the cylinder, this. air can diffuse backward into the cylinder, form NO2, and contaminate the contents of the cylinder. Typical cylinder connection protocols thus call for purging of the pressure regulator and system immediately following connection of the system to the NO gas supply cylinder. While this may eliminate this source of NO2, the causes noted above, and others, remain resulting in NO2 being formed in the gas administration system.
Several approaches have been developed for purging NO gas administration systems prior to use to ensure that NO2 is removed before the administration of NO to the patient begins. For example, U.S. Pat. No. 5,558,083 and European Patent Publication No. 879,612 show the use of a purging valve for system purging purposes. To purge the system, the system is placed in operation to deliver NO gas from the supply source through the system. The purging valve is operated to connect the outlet of the administration system to a gas exhaust rather than to the breathing circuit or other means for delivering NO to the patient. The NO gas moving through the system flushes the NO2 out of the system and into a gas scavenger or the ambient environment. After the purging has been completed, the purging valve is operated to reconnect the outlet of the system to the breathing circuit or other means to deliver NO gas to the patient. The European patent publication further shows the use of the NO delivery control valve in the system for purging purposes rather than a separate purging valve dedicated to purging purposes. The delivery control valve is operated either automatically or through a prompt device alerting the user to purge the delivery system when the administration of NO is commenced. Use of the NO delivery control valve for purging has the advantage of reducing the number of components needed in the NO gas administration system. A disadvantage in utilizing the same control valve and pathway for purging as for delivery is that the patient must be instructed not to use the device until after the purging is complete.
European Patent Publication No. 937479 shows purging of an NO gas administration system if the NO delivery is not triggered by the patient""s breathing within a predetermined time. This prevents the formation of NO2 in the delivery device from the migration of ambient air into the system.
Sulfur hexafluoride (SF6) is used to determine the functional residual capacity of a patient""s lungs and for other pulmonary diagnostic purposes. Small, precisely determined amounts of such an indicator gas are entrained in the inhaled breathing gases to the patient. The amount of indicator gas exhaled by the patient is measured and the exhaled and inhaled gas measurements used in computational techniques that provide the desired diagnostic information. Purging of the delivery line for the indicator gas assists in accurately administering the indicator gas since with purging the gas content of the delivery line is known.
In other cases, it is desired to administer drugs by placing them in the inhaled breathing gases of a patient. Commonly such drugs are in liquid form and are atomized to a fine mist by a nebulizer as they are placed in the inhaled breathing gas. When the drug is to be changed, it is desirable to purge the supply line from the drug reservoir to the nebulizer to ensure the drugs are not mixed.
The present invention provides a technique for purging a fluid administration system of undesired fluids, in a manner that both avoids the need for additional components, such as purging valves, and eliminates the possibility of the patient inhaling the purged fluids. The present invention lends itself to automatic purging of the system on startup and/or before administration of breathing gases to the patient without the need for patient intervention.
In the technique of the present invention, the expiration phase of the patient""s respiratory cycle is sensed. A bidirectional sensor in the breathing gas flow path of the patient may be used to sense the direction of gas flow, and hence the expiration phase of the respiratory cycle. The fluid administration system is operated during the expiration phase to pass the administered fluid through the system for discharge at its outlet. The administered fluid moving through and out of the system carries with it any undesired fluids in the system. However, since the discharge occurs during expiration, the fluids, including the undesired fluids, so removed and discharged from the system to the breathing gases are carried away from the patient with the expired breathing gases. The discharged gases thus do not reach the patient. Once the purging of the system is completed, the supply of fluid during the expiratory phase stops and the normal administration of fluid from the fluid administration system during the inspiratory phase may commence.
To ensure that no fluid discharged during purging reaches the patient, it is preferable that the purging operation be carried out in the initial portion of the expiratory phase of the respiratory cycle. The administered fluid used in purging may be provided as a pulse of fluid in one or more expiratory phases of the patient""s respiratory cycles.
When a respirator is in use with the patient, the outlet of the administration system may be connected to the patient limb of the breathing circuit. The fluid administration system may also be connected to the Y-piece connector of the breathing circuit. Or, the system may be connected to the inspiratory limb of the breathing circuit. A bypass flow from the inspiratory limb to the expiratory limb in the expiratory phase of the respiratory cycle carries off the fluids discharged during purging of the system. The gas flow sensor used to detect the phases of respiratory cycle is typically placed in the breathing circuit. Or, the inspiratory and expiratory phases of the respiratory cycle can be determined from the respirator. The determination of the phases of the respiratory cycle is usually already carried out in fluid administration systems since it is needed for sensing the inspiratory phases in which the administered fluid is supplied to the patient.
The invention can also be utilized when the patient is spontaneously breathing, i.e. without a respirator, with the use of a means to detect the inspiration and expiration phases of the respiratory cycles of the spontaneously breathing patient.
It is preferable that the volume of gas passed through the gas administration system be sufficient to ensure that all undesired fluid is removed from the system. When the administered fluid is a gas, the purging volume needed is determined by the gas volume within the administration system in accordance with the internal volume of the system and prevailing gas pressures. Due to internal gas mixing, the purge volume should be larger than the internal gas volume and preferably at least twice the internal gas volume.
Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the drawings.